Bonding wire production focus and process control

Guide to the whole process control and technical optimization of the bonding wire production process

Introduction: Technical positioning of precision wire manufacturing

As a "neural network" interconnected within a semiconductor package, the diameter accuracy (±0.1 μm) and surface quality (Ra≤0.05 μm) directly determine the reliability of wire bonding. In the manufacturing of ultra-fine wires below 0.1mm, small fluctuations in each step can lead to bond failure – statistics show that 35% of bond failures are due to defects in the drawing process, 28% are related to annealing quality, and 17% are due to improper tension control in the winding process.

As semiconductor packaging evolves to advanced architectures such as chiplets and 3D stacking, bonding wires are facing dual challenges: on the one hand, the wire diameter continues to shrink to less than 15μm (equivalent to 1/5 the diameter of a human hair), and on the other hand, the material system is expanding from traditional gold wire to copper-based and silver-based alloys. This change requires the transformation of the production process from "empirical control" to "digital precise control", and this paper systematically sorts out the technical points and optimization paths of the three core processes of wire drawing, annealing and winding.

1. Wire drawing process: the art of material deformation and precision control

The wire drawing process gradually draws metal billets with a diameter of about 1 mm into ultra-fine wires below 10 μm through the plastic deformation of the mold, and the technical difficulty increases exponentially with the decrease of wire diameter.

1.1 Comparison of equipment technology classification and performance

Contemporary wire drawing equipment has formed a complete technical pedigree, which can be divided into four levels according to processing capacity: processing billets with a wire diameter of > 1.0mm, using multiple passes of continuous stretching (usually 6-8 passes), typical models such as domestic CL-3000, the tensile force can reach 500N, suitable for the preliminary forming of brass, copper and other blanks. The processing range is 0.1-1.0mm, and it needs to be equipped with a tension closed-loop control system (accuracy ±1%). The German Niehoff ML series middle drawing machine is driven by a servo motor, which can achieve stable control of 15-20% of the diameter reduction rate per pass, which is especially suitable for intermediate forming of bonded copper wires. For 0.01-0.1mm wires, the mold accuracy needs to reach ±0.5μm. Kobe Steel's UF series fine drawing machines are equipped with a laser in-line diameter gauge (sampling frequency 1kHz), which can compensate for wire diameter deviations caused by die wear in real time (correction < 0.3μm per hour). For the critical process of handling < 0.01 mm (10 μm), the equipment needs to have a vibration-proof foundation (vibration amplitude < 5 μm) and a constant temperature environment (23±0.5°C). The internationally leading German Schumacher ULT series adopts a magnetic levitation guide system, which can control the wire vibration during the wire drawing process within 2μm.

The choice of lubrication method directly affects the quality of the wire:

The wet drawing system reduces the wire break rate in the ultra-fine drawing stage from 1.2 times/km for the dry process to 0.3 times/km through the "lubrication-cooling-cleaning" trinity. The practice of a semiconductor material company shows that after using imported wet wire drawing machines, the surface scratch rate of bond alloy wires is reduced by 75%, and the bonding yield rate is increased to 99.2%.

1.2 Mold material science and maintenance system

As the "heart" of the wire drawing process, the material selection and maintenance system of the mold varies with the bonding wire material: Polycrystalline diamond (PCD) mold: Made of nanodiamond particles sintered, the hardness can reach HV8000, suitable for hard materials such as palladium-plated copper wire. Its optimal use conditions are: the drawing speed is < 6m/s, the reduction rate per pass is < 18%, and the service life in the processing of palladium-plated copper wire can reach 25km, which is 3 times that of carbide molds.

Natural diamond mold: Processed from single crystal diamond with a purity of more than 99.9%, the surface roughness can be polished to Ra0.01μm, suitable for the ultra-fine drawing process of soft materials such as gold wire and silver wire. However, they are expensive (about 5 times that of PCD molds) and have poor impact resistance, so they need to avoid tensile fluctuations of > 5%.

Establishing a hierarchical maintenance system is key to ensuring mold performance:

Die wear monitoring can be done using acoustic emission technology – when processing palladium-plated copper wire, preventive polishing is arranged when the RMS value of the acoustic emission signal increases from the initial 0.5mV to 1.2mV, while the surface quality of the wire remains at Ra<0.08μm.

1.3 Material adaptability of process parameters

There is a strict matching relationship between the drawing speed and the ductility of the material, and the principle of "the higher the hardness, the lower the speed":

Bond alloy wire: 6-10m/s is recommended in the ultra-fine drawing stage (<25μm), when the plastic deformation of the material is uniform, and the work hardening index is stable at 0.4-0.45. Speeds exceeding 12m/s are prone to "necking", resulting in wire diameter fluctuations of > 0.5μm.

Palladium-plated copper wire: Due to the difference in hardness between the palladium layer (HV180) and the copper matrix (HV110), the speed should be controlled at 3-6m/s. A company found through DOE experiments that 5m/s is the optimal balance point - it can not only ensure the output of 1.2km per hour, but also control the cracking rate of the coating below 0.03%.

Silver alloy wire: Silver's low hardness (HV60) makes it prone to sticking, with a speed set in the range of 3-8m/s, while increasing the lubricant concentration to 8-10% (5% for ordinary wires) to prevent wires from contacting each other using surface tension.

Lubricant management implements a "concentration gradient strategy": rough drawing stage: 8-10% high concentration emulsion with a focus on cooling effect (taking away more than 90% of the heat of deformation), medium drawing stage: 5-7% concentration, balancing lubrication and cleaning function, ultra-fine drawing stage: 2-3% low concentration, reducing residue (controlled at 1-3ppm).

For silver alloy wires, a special formulation containing benzotriazole (BTA) (e.g., patented US8822567B2) is used to prevent oxidation discoloration of silver by forming an adsorption film layer, and experimental data show that the surface oxidation rate can be reduced by 60%.

2. Annealing process: the key to stress elimination and performance regulation

The annealing process eliminates the work hardening caused by the wire drawing process through thermal energy, allowing the wire to obtain the required strength (200-350MPa) and elongation (15-25%) for bonding, and its core is the precise control of the synergy of "temperature-time-protective atmosphere".

2.1 Equipment layout and technical characteristics

The layout of the annealing equipment directly affects the straightness of the wire: Horizontal annealing furnace: simple structure, suitable for thick diameter wire (>50μm). However, gravity causes the wire to sag (up to 50μm at 1m length) and needs to be corrected by a guide wheel set spaced 30cm apart. Vertical annealing furnace: The wire runs in a vertical state, completely eliminating gravity-induced deformation, making it the preferred equipment for ultra-fine filaments (<25μm). Sumitomo Japan's VAF series vertical annealing furnaces are maglev guided (no mechanical contact), which can control the straightness of 15μm silver wire to within 10μm/m.

Modern annealing furnaces have achieved multi-zone temperature precise control (±1°C), and the typical three-zone temperature control scheme is: preheating zone: the temperature is set to 60-70% of the target annealing temperature, the lubricant residual on the surface of the wire (mainly mineral oil components) is removed, and the annealing zone: the core reaction zone is set to 180-450°C according to the material (gold wire 300-350°C, copper wire 400-450°C).Cooling zone: Rapid cooling with inert gas (cooling rate 5-10°C/s) to prevent excessive grain growth

Scientific ratio of process parameters

The composition of the protective atmosphere is critical to the quality of annealing, and a typical 95% N₂+5% H₂ gas mixture has a dual effect: hydrogen (reducible): eliminates oxide layers on the surface of the wire (e.g. CuO, Cu₂O of copper), detected by thermogravimetric analysis (TGA), reduces the oxygen content from 0.15% to less than 0.03%, nitrogen (inert): as a carrier gas, Maintain a slightly positive pressure (50-100Pa) in the furnace to prevent outside air intrusion, and the gas flow rate needs to match the running speed of the wire, following the "linear speed - flow" formula: Q=k×v×d², where k is the material coefficient (copper wire 1.2, silver wire 0.9), v is the linear speed (m/min), and d is the wire diameter (mm). When a 15μm gold wire is running at 10m/min, the optimal flow rate is 3-4L/min, where annealing uniformity (σ≤3%) is optimal. The matching of annealing temperature to take-up speed follows the principle of "conservation of energy": for a 25μm diameter bonded wire, the take-up speed can be increased from 8m/min to 12m/min when the annealing temperature is increased from 300°C to 350°C (maintaining the same thermal energy input). Excessive energy input can lead to grain coarsity (from 50nm to 200nm), resulting in wire elongation exceeding 30%, which is not conducive to bond synthesis.

Annealing strategies for special materials

Annealing of copper-based bonded wires requires stricter atmosphere control: Palladium copper wire: Hydrogen ratio needs to be increased to 8-10% to prevent oxidation of palladium (PdO causes a 40% decrease in bond strength), bare copper wire: Organic protective film (thickness 2-5nm) is applied immediately after annealing, and the carbon-oxygen ratio (C/O) of the protective film is >confirmed by XPS analysis to be 3.0, which can be stored in air for 72 hours without oxidation

The annealing of silver alloy wire is prone to the phenomenon of "hydrogen embrittlement", and the solutions include: 1Reduce the proportion of hydrogen to 3%, 2Add the vacuum treatment process after annealing (-0.09MPa, 30min). 3. Adopt segmented cooling (slow cooling to 200°C first, then rapid cooling).

3. Winding process: precision winding and tension management

The winding process neatly winds the annealed wire on a reel (usually 500m or 1000m gauge), and its quality directly affects the pay-off stability of downstream bonding equipment - statistics show that a >5% fluctuation in winding tension can increase the bond breakage rate by 3 times.

3.1 Comparison of equipment technology evolution and performance

Winding equipment has evolved from traditional stationary take-up to an intelligent adjustable system:

The core advantage of the intelligent compensation system is the real-time monitoring of the cable position using a laser profile sensor (sampling frequency 500Hz).

, servo motor-driven lateral adjustment mechanism (response time < 50ms), tension controller based on fuzzy PID algorithm,

According to the production data of a packaging material company, after upgrading the intelligent winding system, the unwinding success rate of 15μm gold wire has increased from 85% to 99.2%, and the effective utilization rate of each roll of wire has increased by 12%.

3.2 Material science of guide wheel systems

The guide rollers, as a key component of wire contact, are selected according to the principle of "hardness matching": PTFE guide wheels: Shore D 55 hardness for coarse wires > 50 μm. Its low coefficient of friction (0.04) reduces scratches on the wire surface, but requires monthly wear testing (allowed groove depth < 5μm). POM Guide Wheel: Hardness up to HV800 after chrome plating for wires in the 20-50μm range. Surfaces should maintain a mirror finish of Ra≤0.2 μm and should be ultrasonic cleaned with isopropyl alcohol (40 kHz, 5 minutes) weekly to remove lubricant residues. Ceramic guide wheel: Made of zirconia (ZrO₂) material, with a hardness of HV1200 or higher, designed for < 20μm ultrafilament. The formation of a 0.5μm oil reservoir on the surface through laser microtexture technology can increase the lubrication effect by 50%, while controlling the micro-scratch rate to less than 0.3 /m.

The maintenance of guide wheels implements a "three-level inspection" system:

1. Daily: Visually inspect the surface for obvious damage

2. Weekly: roundness meter detection (allowable deviation ≤2μm).

3. Monthly: White light interferometer re-test roughness (no more than 120% of the initial value).

3.3 Material adaptability of tension control

The sensitivity of wires of different materials to tension is significantly different:

Gold wire: good ductility (20-25%), can withstand large tension fluctuations (±8%), recommended tension value 0.5-1.0cN (corresponding to 25μm wire diameter), copper wire: high rigidity (elongation 15-20%), tension needs to be stable at 0.8-1.2cN, fluctuation control within ±5%, silver wire: easy to produce plastic deformation, tension is set to 0.4-0.7cN, and the winding process needs to be linearly decreasing (5% reduction per 100m), the tension control adopts a "segmentation strategy": initial winding (0-200m): higher tension (110% of the set value) to ensure tight fixation of the wire, middle (200-800m): standard tension, constant,End (800-1000m): Reduced to 90% of the set value to prevent slack at the end of the wire, for ultrafine filaments with a diameter of < 20μm, a tension sensor (accuracy ±0.01cN) and a servo feedback system are required, and the sampling frequency is not less than 100Hz, ensuring a tension fluctuation of <3% during the acceleration/deceleration phase.

4. Whole process quality control and technology trends

4.1 Key process parameter monitoring system

Establish a real-time monitoring network covering three major processes:

A leading company implemented this monitoring system to increase the CPK value of the production process from 1.33 to 1.67, and the product defect rate from 300ppm to less than 50ppm.

Future technology development direction

Bonded wire production technology is evolving in three directions: Intelligent manufacturing: Digital twin system: simulates drawing die wear and wire deformation in virtual space, with a prediction accuracy of up to 92%, adaptive control: real-time optimization of wire drawing speed and annealing temperature based on AI algorithms (such as random forests) to expand the process window by 40%, and closed-loop recovery of lubricant: 95% through nanofiltration technology The above emulsion recycling reduces the amount of hazardous waste disposed of, and the low-hydrogen annealing process: development of a new catalyst to reduce the amount of hydrogen from 5% to 2% while maintaining the same reduction effect, development of gradient hardness mold (surface HV10000, matrix HV3000) for copper alloy nano-coated wires, optimization of gold - Annealing regime of silver composite wires to determine the optimal temperature-time curve by synchronous thermal analysis (STA).